This invention relates to greenhouses or similar structures which require the removal of large quantities of heat flux. Specifically, this invention relates to a method and structural means to utilize external wind energy, both pressure and flow, as well as to produce and control both air pressure and air flow within an air-supported structure, such as an air-supported greenhouse.
To date, commercial greenhouses are constructed with framing covered with a transparent or translucent, air and water impermeable material. The primary function of the frame is to support the covering material against its own weight, wind, rain, and potential snow loads. The frame also supports doorways for passage into or out or the greenhouse. Often the frame supports means for ventilation. This ventilation may be passive, by way of external wind and/or thermal convection, or active ventilation via a fan or fans.
One particular version of commercial greenhouses consists of attaching two layers of plastic film to a frame thereby sealing the edges of the two layers together. Air, usually from the outside in order to avoid condensation, is forced between the two layers of plastic with a small blower, thereby separating the two layers and maintaining tension in the film. This technique was developed in 1964 and is an improvement over single layer structures in two ways. First, it provides an insulating air space between the two layers of plastic film. Secondly, it reduces the likelihood of whipping, then ripping, of the plastic film in the wind, thereby increasing the life of the film. Disadvantages with this system are the cost of an extra layer of plastic film and blower and the reduced transmission of light, 10% or more reduction, into the greenhouse. Another potential disadvantage is that, depending on the inflation blower's position, orientation, and surroundings, the dynamic pressure of the external wind can add to or subtract from the air pressure between the two layers of plastic film. For wind conditions that lead to a reduction in the internal air pressure, this effect can potentially lead to whipping and ripping of the plastic film. For wind conditions that lead to an increase in the internal air pressure, this effect can potentially lead to over pressurization and ripping of the plastic film.
For all current versions of commercial greenhouses, a frame has been required to support the plastic film or other glazing. While the frame serves some use, it is expensive and blocks a portion of the sunlight from reaching the plants in the greenhouse. Due to these deficits, alternatives have been sought to replace the framing material. One experimental approach has been to replace the typically wood or metal frame with an air pressurized frame, or air-beams. These frames, usually of transparent plastic, are flaccid without internal air pressurization. Upon pressurization, they become rigid and take their intended shape. U.S. Pat. No. 2,854,014 to Hasselquist (1958) describes an “Inflatable Shelter” supported by inflated tubing. U.S. Pat. No. 4,856,228 to Robinson, Sr. (1989) also describes a “Tunnel system for care or seeds, plants and the like” supported by inflated tubing.
An alternate approach has been to integrate the inflatable frame into the covering so that essentially the entire light-transmitting surface is made rigid by pressurized air. This approach also provides an insulating air space between the layers of covering. U.S. Pat. No. 4,160,523 to Stevens (1979) and U.S. Pat. No. 6,061,969 to Leary (2000) both utilize this strategy.
The term “air-inflated structure” describes the above mentioned inflatable structures, where the shape of the structure is produced by air pressurized tubes or cells, while the air within the enclosed space of the structure remains unpressurized. Transparent, air-inflated structures, when used as a greenhouse, still have the same requirements for ventilation, heating, and cooling as conventional greenhouses.
The alternate design, the “air-supported structure,” is comprised of a membrane, which is anchored and sealed to a wall or the ground at the edges, and air pressurization means within the enclosed space of the structure to keep the membrane suspended and taught. Airlocks are usually attached for ingress and egress. Air-supported structures have critical issues with respect to pressurization and ventilation, which are required to maintain the integrity and inhabitability of the structure. Additionally, both pressure and ventilation effect the energy consumption of the structure.
Methods and apparatus have been developed to control internal pressures based on external wind, snow, and ice loads as well as the air-supported membrane's height and tension. The goal is to maintain the minimal internal pressure, relative to the external pressure, necessary to maintain the structural integrity of the membrane and its height, i.e. clearance above the floor. Minimalizing the pressure differential between inside and outside minimalizes the tension within the membrane and the energy requirements for inflation. Therefore, sensors and controls have been sought to maintain idea pressurization. U.S. Pat. No. 2,948,286 to Turner (1960) utilizes mechanical means, which measure membrane height, side wall erectness, or membrane tension, to activate a switch which controls the operation of an inflation blower thereby controlling the internal pressure and the resulting membrane height, side wall erectness, and membrane tension. However, no exhaust vents are included in this invention as ventilation is not within the scope of this particular invention. U.S. Pat. No. 3,159,165 to Cohen, et al. (1964) utilizes mechanical means, which measures the structure's girth to activate a switch which controls the operation of an inflation blower thereby controlling the internal pressure and the resulting structure's girth. Here again, no exhaust vents are included in this invention as ventilation is not within the scope of this particular invention. U.S. Pat. No. 4,936,060 to Gelinas, et al. (1990) utilizes radar means, which measures the membrane's height, to control the operations of an inflation blower and exhaust vent, thereby controlling the internal pressure and volume and the resulting membrane's height U.S. Pat. Nos. 5,685,122 to Brisbane, et al. (1997) and 6,032,080 to Brisbane, et al. (2000) utilize sensor means, which measure eternal wind speed and potential conditions for snow and ice loading, to control the operation of an inflation blower and exhaust ventilation thereby controlling the internal pressure.
In the two patents to Brisbane, et al., only one location is specified for the air pressure sensor, namely within the air-supported structure. The pressures specified in the process control element of the invention are differential pressures, not absolute pressures. That is to say that the specified static pressures of 0.4″-1.4″ of water column are much less than atmospheric pressure and really represent the difference between inside and outside pressures, or more precisely outside static pressure, which discounts the dynamic pressure of the wind. Therefore, there would need to be a sensor external to the structure to sense the outside static pressure. There is a “wind velocity sensor” indicated on top of the structure which is stated as an anemometer, but there is no description of linking this sensor to the static pressure sensor disclosed in the patent. Also, only wind speeds without respect to wind direction are described relative to the wind sensor.
Ultimately, the position and orientation of a necessary outside static pressure sensor is critical to the successful operation of the invention being disclose. For example, a wind speed of 25 MPH has a dynamic pressure of 0.3″ of water column. Under these conditions, the external pressure on the air-supported structure's surface, as would be the case in a domed structure indicated in the figure, can vary from “atmospheric pressure plus 0.3″ of water column” on the upwind side to “atmospheric pressure minus 0.3″ of water column” on the side (between upwind and downwind) as well as the top. At 50 MPH, these variations quadruple to +/−1.2″ of water column. Thus, if the outside static pressure sensor were on the top or side (relative to the wind) of the structure and the internal pressure, which is uniform within the structure, were 1.4″ of water column above the outside pressure sensed, then the upwind side of the structure would collapse as the external pressure would be 1.0″ of water column pressure above the internal pressure (1.2″+1.2″−1.4″). Therefore, this invention, unless appropriately modified, is destined to fail.
The second critical issue for air-supported structures is ventilation. Of air-supported structures, most have been made with low-transparency, high-reflectivity membranes to reduce the radiant energy flux into and out of these structures, whereby reducing the cooling and heating requirements of the structure. These structures have covered tennis courts, pools, athletic fields and stadiums. Ventilation is required to remove the reduced solar radiant energy flux and/or occupant emissions, whichever is greater. Ventilation requirements under these conditions are much less (80%-90% less) than ventilation requirements for greenhouses, where the nearly full solar radiant energy flux needs to be removed from the greenhouse in order to prevent overheating.
Ventilation necessary to meet the summertime cooling requirements for greenhouses is approximately 10 cubic feet per minute for every square foot of floor space. In a frame-supported greenhouse, this is achieved with relatively low pressure (up to 0.25″ of water column) propeller fans, as opposed to high pressure blowers used in air-supported structures. Under typical summertime conditions, a 48″ diameter propeller fan with a 1 hp motor is sufficient to ventilate and cool approximately 2,000 square feet of floor space.
For a typical air-supported structure, a 25″ wheel diameter blower with a 10 hp motor would be required to produce the same ventilation, because of the higher pressure within the air-supported structure compared to the frame-supported structure. Therefore, scaling the present air-supported structure technology to a greenhouse facility would require 10 times the electrical power.
One manufacturer of air-supported structures, Environmental Structures Incorporated, has addressed this issue by replacing the high power blowers with medium pressure (up to 0.5″ of water column) propeller fans. Under relatively calm conditions, these fans require 2-3 times the power of the frame-supported greenhouse fans. In the event of windy conditions, a tandem fan, two medium pressure fans placed in series, is turned on, thereby doubling the pressure, which is necessary to support the membrane against the dynamic pressure of higher winds. The net result is that the electrical energy consumption of this system is about 3 times that of a typical greenhouse. Because of this higher operating cost and an initial cost twice that of conventional greenhouses, these air-supported greenhouses have seen only individual use.
One way to reduce this energy consumption has been to reduce, or almost eliminate, the ventilation. Individual examples are at:
http://www.actahort.org/books/42/42 5.htm and
http://www.sunset.com/sunset/Premium/Garden/1997/02-Feb/Greenhouse297/Greenhouse297.html and
http://savagefarmer.blogspot.com/2006/02/big-greenhouse.html
Each of these greenhouses lacks the ventilation necessary to sufficiently cool them in the summer as stated in the each of the above sources.
Yet another attempt to overcome these deficiencies was made by the University of Arizona Environmental Research Laboratory in cooperation with the University of Sonora, Mexico. In 1968.air-supported greenhouses were pressurized by a blower but not ventilated. The cooling was accomplished by drawing air through a packed-column heat exchanger which was sprayed with cool saltwater from the nearby sea. This system, however, kept the relative humidity in the greenhouse near 100%, which is not ideal for many plants.
Within the scope of the invention, three other examples of prior art are worth mentioning. U.S. Pat. No. 3,924,364 to Eerkens (1975) describes a “Wind-Inflatable Tent.” Here the wind's dynamic pressure is utilized to pressurize, but not ventilate, an air-supported tent. An externally supported opening to the tent is pointed into the wind. No means for tracking the wind are provided. The tent is utilized as a means for escape from the wind, i.e. shelter.
Another shelter, described in U.S. Pat. No. 6,070,366 to Pierson (2000), includes a rotatable air scoop with a wind wane on the opposite side of the air scoop's opening, apparently to automatically turn the opening into the wind. A louvered vent is just inside the opening of the air scoop and oriented to only allow air from the outside wind into the air scoop and then into the shelter. The louvers only operate and allow air into the shelter when the total (dynamic plus static) pressure of the wind somewhat exceeds the internal air pressure. A plurality of these air scoops would be attached to the roof fabric of the shelter. The function of these rotatable air scoops is to allow for additional inflation as a result of local wind conditions over the surface of the shelter. The air scoops are not the primary means of inflation for the shelter. These air scoops are not connected to any mechanical means, such as a blower or fan, to inflate the shelter. Also, no exhaust vents are included in this invention as ventilation is not within the scope of this particular invention.
Lastly, Tom Elliot describes a passive air conditioning system using a rotatable air scoop directing wind flow through evaporative cooling pads and into a house in order to cool the house, but not to pressurize it. The description is located at:
http://www.thedisease.net/arcana/survival/How-To Survival Library/library2/aircool.htm
As the system is passive, no fans are used to augment the flow, which is created by wind and the natural convection of the cooled air flowing down a tower and into the house.
In conclusion, I am not aware of any air-supported or frame-supported structure that provides a method or apparatus for directing wind energy, i.e. pressure and flow, into any mechanical ventilation device, such a fan or blower, thereby ventilating and potentially pressurizing the internal space of a structure utilizing the energies of both the wind and the mechanical ventilation device. In addition, I am not aware of any air-supported or frame-supported structure that provides a method or apparatus for determining or directing the outside dynamic wind pressure and outside static pressure, independently, to control an exhaust vent, thereby regulating the internal pressure of the structure relative to the outside dynamic wind pressure and outside static pressure.